Mobile radio communication systems may be used for various kinds of communication. Depending upon intended major use and required services many more or less different mobile radio communication systems have been suggested or are in operation.
When mobile radio communication systems may have a large number of base and mobile stations efficient use of radio frequency spectrum and traffic handling capacity become very important. Kinds of communication and services provided and the ability to establish and uphold an established connection are also important.
Some cellular mobile radio systems use cell patterns and radio channel reuse rules with fixed allocation of radio channels to individual cells and serving base stations. To improve flexibility of some cellular mobile radio system it has been suggested not to allocate all available radio channels to individual cells and base stations according to a fixed cell pattern and reuse rule but to allocate some of the avilable radio channels differently from time to time. Depending on traffic load as well as other circumstances some of the channels are individually allocated to individual cells and base stations for a limited time. It has also been suggested not to allocate any available radio channels permanently to individual cells or base stations but to allocate all available radio channels one by one according to prevailing traffic conditions to base stations.
In mobile radio communication systems using time division multiplexed radio channels not only radio channel but also time slot should be determined for each call at set up and handover. In some systems the decision on radio channel and time slot is more or less centralized and done in the fixed part of the system, e.g. in base station or mobile switching centre. In other systems the decisions are decentralized and each mobile station decides which base station, radio channel and time slot to be used for a particular call.
In a cellular mobile radio system with many base stations and many mobile stations it is important to have procedures enabling the system to keep track of mobile stations, choose base stations and select radio channels for calls in a convenient way. This is important irrespective of whether the cellular mobile radio system is intended to be used for speech communication only or primarily or if the cellular mobile radio system is intended to be used for transmission of data only or mostly. However, due to different fields of use, number of base and mobile stations, allocation of channels and performance requirements a roaming or handover procedure very convenient in one cellular mobile radio system may not be particularly convenient in an other cellular mobile radio system.
One aspect of roaming, call set up and handover procedures to consider is the distribution of procedure work load between mobile stations, base stations and the rest of the system, e.g. mobile switching centers. Some known roaming and handover procedures require comparatively much work by base stations and comparatively little work by mobile stations. In other known roaming and handover procedures the work load on base stations is comparatively low but the work load on mobile stations is comparatively high.
Another aspect of procedures to consider is the amount of signalling required on the radio channels between base and mobile stations and the amount of signalling required in the rest of the system, e.g. between base stations and mobile switching centre, e.g. for the purpose of roaming or handover. Some cellular mobile radio communication systems may be able to handle comparatively large amount of such signalling conveniently whereas for other cellular mobile radio systems a large amount of such signalling could be detrimental for the overall performance.
A further aspect of procedures to consider is the limited power available at some mobile stations, in particular small light-weight battery powered portable stations. Normally a mobile station consumes comparatively much power when transmitting radio signals and substantially less power when receiving radio signals. A portable station may be designed to consume very little power when switched on but neither transmitting nor receiving radio signals. In order not to consume more energy than necessary it has been suggested that mobile stations may be in an idle state when not involved in an ongoing call or setting up a call. In the idle state a mobile station may be unable to transmit and receive radio signals during a major part of a repeated time interval and be able to receive but not transmit radio signals only during a minor part of the repeated time interval. The average power consumption may be reduced when the minor time interval is reduced in relation to the major time interval. If the portable station shall be able to perform necessary roaming in the idle state the minor time interval may not be made smaller than necessary to receive radio signals required for roaming. If small light-weight battery powered portable stations shall be frequently used in a mobile radio communication system it is important that procedures for roaming and set up of calls take into account the restriction caused by necessary power consumption of such stations and allow idle states. Portable stations may be rapidly moved when in idle state and the procedures must allow quick roaming and call set up even at mobile stations in an idle state.